Can I access the render screen size in a custom filter 2D in the game engine?
All the game properties stored in the object that calls the Filter 2D are passed to the shader as uniforms. So if you have a width and a height float game properties you can set their values at run-time to match the window size.

However, be aware that the window size (which you get from render.getWindowWidth() or render.getWindowHeight()) is not the buffer size. The buffersize is the closest power of two for that dimension (e.g., a window that is 1024 x 768 will have as buffers 1024 x 1024).